I've been thinking of making a big list of all the great ideas in ODST that haven't been finished/properly looked into. I'll get to work on it soon.
I know of three major ones.
Oni Alpha Site ChargeA lone charge (detonator) was left in the bottom of one of the chutes when you get to the part where you play firefight in.
We had managed to activate this switch by jumping down and dying next to it. It turns green and we can hear the click on sound. But we cannot continue with this switched, because you have to die to get it. Or maybe only one person does, I can't remember.
I suggest we find any other switches, and do a legendary run and hit all the switches.
ZombielitesWell, you all know a lot about this one. Dead elites coming to life. Why is this model left in the game, taking valuable space? Well, lets figure that out.
ScattingVery strange mystery. Jumping atop of the squidhenge pillars triggers the engineers to start Scatting, or start floating from one forerunner light to another.
Night, I will be prioritizing ODST unless a reach phenomenon comes up. I will report if anything has changed.